Methods, devices, and systems for communicating with an implantable medical device of a last far field communication session during a subsequent far field communication session while using a same session key

ABSTRACT

An external device transfers a key to an implantable medical device over a proximity communication and then establishes a first far field communication session with the implantable medical device where the key is used for the first communication session. This first communication session may occur before implantation while the implantable medical device is positioned outside of the sterile field so that using a proximity communication is easily achieved. Once the implantable medical device is passed into the sterile field for implantation, the external device may then establish a second far field communication session with the implantable medical device where the last key that was used for the first communication session is again used for the second communication session which avoids the need for another proximity communication to occur within the sterile field.

RELATED APPLICATIONS

The present application is a continuation application of U.S.application Ser. No. 15/794,226, filed on Oct. 26, 2017, which is adivisional application of U.S. application Ser. No. 14/695,037, filed onApr. 23, 2015, which claims priority to U.S. Provisional Application No.61/983,886, filed on Apr. 24, 2014, and having the title METHODS,DEVICES, AND SYSTEMS FOR COMMUNICATING WITH AN IMPLANTABLE MEDICALDEVICE OF A LAST FAR FIELD COMMUNICATION SESSION DURING A SUBSEQUENT FARFIELD COMMUNICATION SESSION WHILE USING A SAME SESSION KEY.

TECHNICAL FIELD

Embodiments relate to systems where an external device communicates withan implantable medical device via a far field communication session.More particularly, embodiments relate to systems where the externaldevice communicates with an implantable medical device that is the lastdevice to which the external device has previously communicated whileusing the same session key.

BACKGROUND

Implantable medical devices (IMDs) provide electrical stimulation and/ordrug therapy to patients to address medical issues. The IMD is implantedwithin the body of the patient at a convenient location such as withinthe abdomen or within the upper torso. An implantable medical lead iscoupled to the IMD and may be routed from the site of the implantablemedical device to a target site where the electrical stimulation isprovided to the tissue, such as within the spinal column, within thebrain, and so forth. The IMD generates electrical stimulation signalsthat are carried by an electrical conductor within the lead toelectrodes located at a distal region of the lead that are positioned atthe target site.

In order to implant the IMD, a surgical procedure is used where asterile field is provided about the body. Prior to passing the IMD intothe sterile field, an external device may open a far field communicationsession with the IMD in order to perform preliminary tasks includingtransferring patient information to the IMD. For instance, the externaldevice may open a MICS band communication session for this purpose.Often during this time, the patient is being prepped for the surgicalprocedure within the sterile field. Once the preliminary tasks arecompleted by the external device, the external device may then terminatethe communication session. Closing the communication session preservesthe battery life of the IMD by allowing the far field communicationcircuit to sleep. The IMD is then passed into the sterile field and issurgically implanted within the body of the patient.

Once the IMD has been implanted, the external device is then used toopen another far field communication session with the IMD. The externaldevice may communicate with the IMD to initiate integrity checks of theleads that are connected to the IMD. The external device may alsocommunicate with the IMD to program the stimulation therapy parameters.The communication session may then be closed and the surgical procedureis concluded.

Because the external device utilizes far field communications with theIMD, it is desirable to ensure that the external device is communicatingwith the intended IMD. Other IMDs may also be in range of the far fieldsignals of the external device, and the external device should avoidopening far field communication sessions with these other IMDs. Toensure that the external device is communicating with the correct IMD,the external device may use a proximity communication such as a nearfield inductive communication via a near field wand that only theintended IMD is able to receive. The proximity communication maytransfer a unique key from the external device to the IMD. The externaldevice may then ensure that the key is being used by the IMD that isresponding via the far field communications such as for encryptionpurposes or to otherwise identify the communications from the IMD.

This transfer of the key via the proximity communication ensures thatthe external device is communicating with the correct IMD. However,because the external device of this scenario is not permanently bondedto the IMD, the external device attempts to exchange a new key each timea far field communication session is attempted. Therefore, when theexternal device attempts to start a far field communication session withthe IMD while the IMD is located within the sterile field, the proximitycommunication requires that the proximity wand be introduced into thesterile field. This introduces additional burdens and delay into theimplantation process.

SUMMARY

Embodiments address issues such as these and others by providingexternal devices that utilize a key provided to a last device for whicha far field communication session was established when an attempt toexchange a new key via a proximity communication fails. Therefore, whenthe external device attempts to communicate with the IMD once the IMD isin the sterile field, the proximity wand may be kept outside the sterilefield such that the proximity communication fails. The external devicethen attempts to start the communication session where the key used inthe last communication session is again used in the presentcommunication session. Because the IMD has retained the key used in thelast communication session that occurred while the IMD was outside thesterile field, the IMD is able to enter the communication session withthe external device while the IMD is in the sterile field without theIMD receiving a new key.

Embodiments provide a method of communicating with an implantablemedical device. The method involves exchanging a key with theimplantable medical device over a first proximity communication. Themethod further involves beginning a first far field communicationsession with the implantable medical device and exchangingcommunications during the first far field communication session with theimplantable medical device where the implantable medical deviceimplements the key when exchanging the communications. The methodinvolves terminating the first far field communication session andattempting to exchange a second key over a second proximitycommunication after terminating the first communication session.Additionally, the method involves attempting to begin a second far fieldcommunication session where the external device expects communicationsof the second session, including a response to the attempt, to use thesecond key. If no response is received to the attempt to begin thesecond far field communication session which uses the second key, thenthe method further involves attempting to begin the second far fieldcommunication session where the external device expects communicationsof the second session, including the response to the attempt, to use thefirst key. If a response that uses the first key is received to theattempt to begin the second far field communication where the externaldevice expects the response to use the first key, then the methodfurther involves conducting the second far field communication sessionwhere communications of the second session use the first key.

Embodiments provide an external device that communicates with animplantable medical device. The external device includes a proximitycommunication circuit, a far field communication circuit, and acontroller coupled to the proximity communication circuit and the farfield communication circuit. The controller is configured to exchange akey with the implantable medical device over a first proximitycommunication. The controller is also configured to begin a first farfield communication session with the implantable medical device andexchange communications during the first far field communication sessionwith the implantable medical device where the implantable medical deviceimplements the key when exchanging the communications. The controller isfurther configured to terminate the first far field communicationsession, attempt to exchange a second key over a second proximitycommunication after terminating the first communication session andattempt to begin a second far field communication session where thecontroller expects communications of the second session, including aresponse to the attempt, to use the second key. Additionally, thecontroller is configured such that if no response is received to theattempt to begin the second far field communication session that usesthe second key, the controller then attempts to begin the second farfield communication session where the controller expects communicationsof the second session, including the response to the attempt, to use thefirst key. If a response that uses the first key is received to theattempt to begin the second far field communication where the controllerexpects the response to use the first key, the controller then conductsthe second far field communication session where communications of thesecond session use the first key.

Embodiments provide a system that includes an implantable medical deviceand an external device. The external device includes a proximitycommunication circuit, a far field communication circuit, and acontroller coupled to the proximity communication circuit and the farfield communication circuit. The controller is configured to exchange akey with the implantable medical device over a first proximitycommunication. The controller is also configured to begin a first farfield communication session with the implantable medical device andexchange communications during the first far field communication sessionwith the implantable medical device where the implantable medical deviceimplements the key when exchanging the communications. The controller isfurther configured to terminate the first far field communicationsession, attempt to exchange a second key over a second proximitycommunication after terminating the first communication session andattempt to begin a second far field communication session where theexternal device expects communications of the second session, includinga response to the attempt, to use the second key. Additionally, thecontroller is configured such that if no response is received to theattempt to begin the second far field communication session that usesthe second key, the controller then attempts to begin the second farfield communication session where the external device expectscommunications of the second session, including the response to theattempt, to use the first key. If a response that uses the first key isreceived to the attempt to begin the second far field communicationwhere the external device expects the response to use the first key, thecontroller then conducts the second far field communication sessionwhere communications of the second session use the first key.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an operating environment for embodiments of the medicalsystem where the IMD is present outside of the sterile field.

FIG. 2 shows the operating environment for embodiments of the medicalsystem where the IMD is present inside of the sterile field.

FIG. 3 shows an example of components for embodiments of an externaldevice.

FIG. 4 shows an example of components for embodiments of an IMD.

FIG. 5 shows an example of logical operations for embodiments of theexternal device to communicate with the IMD during the implantationprocedure.

FIG. 6 shows an example of logical operations for embodiments of theexternal device to establish a far field communication session with theIMD.

DETAILED DESCRIPTION

Embodiments allow an external device to exchange a key with an IMD whilethe IMD is outside of a sterile field by using a proximity communicationand then use that key for a far field communication session with the IMDwhile the IMD remains outside of the sterile field and also for a secondfar field communication session with the IMD while the IMD resideswithin the sterile field. Thus, no proximity communication with the IMDis necessary while the IMD is within the sterile field in order toestablish the far field communication session with the IMD.

FIG. 1 shows an example of an operating environment for the variousembodiments. In this example, a system 100 includes an external device102 and includes an IMD 104 that is being implanted into a patient 118.For the stage of the implantation procedure shown in FIG. 1, theexternal device 102 and the IMD 104 are in an area 114 outside of asterile field area 116. While in the area 114 that is outside of thesterile field, initial preparation of the IMD 104 may be performedincluding transferring information about the patient 118 including anidentification of the patient. The patient 118 is positioned within thesterile field area 116 and is being prepped for the implantation of theIMD 104 at this point. While the IMD 104 is located outside of thesterile field, the IMD 104 may be contained within a sterile packagingwhich does not interfere with the near field and far fieldcommunications. The IMD 104 is then removed from the sterile packagingwhen being introduced into the area 116 of the sterile field.

The external device 102 may initially communicate with an externalstimulator 120 that may have previously been installed for the patient118 for a trial period of stimulation therapy. The external device 102may establish a communication link 122 with the external stimulator 120which may be wired or wireless and may be near field or far field whenwireless. The link 122 may be established while the external stimulator120 remains attached to the patient 118 or may be established once theexternal stimulator 120 has been removed from the patient 118 and passedinto the area 114 outside of the sterile field. The external device 102may obtain patient information, therapy programming, and the like fromthe external stimulator 120.

The external device 102 also communicates with the IMD 104 while the IMD104 is present in the area 114 outside of the sterile field. Theexternal device 102 sends far field wireless signals 110, such as in themedical implant communication service (MICS) band, to the IMD 104 duringa communication session. However, in order to establish thecommunication session, the external device 102 utilizes a proximitycommunication 108, such as a near field inductive coupling from a wandor hoop 106 positioned in close proximity to the IMD 104 to send aunique key to the IMD 104. Because the near field communication has avery limited range, only the intended IMD 104 is in range to receive theproximity communication containing the unique key. Therefore, when theexternal device 102 attempts to begin the far field communicationsession by sending the far field signals 110, the intended IMD 104 isthe only IMD that can respond with far field communication signals 112that utilize the unique key to identify the far field signals 112 asbeing from the intended IMD 104. For instance, the key may be used forencryption, as a header, or in any other manner that identifies the farfield signals as being from an IMD possessing the unique key.

The external device 102 then proceeds to communicate with the intendedIMD 104 via the far field signals 110, 112 to exchange information withthe IMD 104. The information being exchanged may be basic setupinformation including the patient information. As mentioned above, thisinformation for setup may have been obtained from the externalstimulator 120 when present and/or may be entered by a user manually viaa user interface at the external device 102. The external device 102then closes the far field communication session with the IMD 104 suchthat the IMD 104 may turn off the far field communication circuits toconserve battery power.

The next stage of the implantation procedure involving the system 100′involves the IMD 104 which has been moved into the sterile field area116 being implanted into the patient 118. This stage is shown in FIG. 2.Once the IMD 104 is implanted, a second far field communication sessionbetween the external device 102 and the IMD 104 is established. Theexternal device 102 is for use by clinicians and is used to communicatewith any IMD being implanted rather than being permanently bonded to aparticular IMD. Therefore, the external device 102 looks to exchange aunique key for each far field communication session via a proximitycommunication 108′. However, at this stage of the implantationprocedure, the proximity wand or hoop 106 remains in the area 114outside of the sterile field and therefore the IMD 104 is no longerwithin range of the proximity communication 108′.

Rather than bringing the wand or hoop 106 into the sterile field area116, the external device 102 retains the last used key so that it may beused in a subsequent communication with the same IMD 104 and attempts touse the last key when doing so is appropriate. Before attempting to usethe last key for another session, the external device 102 first attemptsthe proximity communication 108′ to exchange a new unique key and thenattempts to start the second far field communication session of theimplantation procedure via far field signals 110 based on the new key.The external device 102 requests a far field response from the IMD 104where the external device 102 expects the new key to be used. However,the IMD 104 does not have the new key and cannot properly respond to therequest for a response by the external device 102. The external device102 reacts to the failure to receive a response from the IMD 104 thatutilizes the new key by requesting a far field response from the IMD 104where the external device 102 expects the last key used by the externaldevice 102 to be used in the response.

Where the IMD 104 is the same IMD that communicated with the externaldevice in the immediately prior far field communication session, whichwill be the case in the implantation procedure scenario being shown inFIGS. 1 and 2, the IMD 104 has the last key used by the external device102. Therefore, the IMD 104 can properly respond to the request by theexternal device 102 to respond using the last used key. The secondcommunication session then proceeds with the external device 102 sendingfar field signals 110 from the area 114 outside of the sterile field andthe IMD 104 sending far field signals 112 from the area 116 within thesterile field.

FIG. 3 shows an example of components for embodiments of the externaldevice 102. The external device 102 includes a controller 130 thatperforms logical operations, such as those of FIGS. 5 and 6 which arediscussed below, in order to establish the communication sessions withthe IMD 104. The controller 130 may be of various forms such as ageneral purpose programmable processor, an application specificprocessor, hardwired digital logic, or combinations thereof.

The controller 130 communicates with other components of the externaldevice 102. For instance, the controller 130 may be linked to a userinterface 132, such as an input device that a user may manipulate toprovide commands and information to the controller 130. The userinterface 132 may include features such as a keyboard, a touch screen, acomputer mouse, or combinations thereof.

The controller 130 also communicates with a proximity communicationcircuit 134 in order to provide the proximity communications 108 to sendthe unique key to the IMD 104. The proximity communication circuit 134may include a coil tuned to an appropriate frequency for near field,inductive coupling and also include a driver to pass electrical signalsthrough the coil. The proximity communication circuit 134 may beunidirectional for sending signals to the IMD 104. Alternatively, theproximity communication circuit 134 may be bi-directional forembodiments where the IMD 104 also has a bi-directional proximitycommunication circuit which allows the external device 102 to receiveproximity communications sent by the IMD 104.

The controller 130 communicates with a far field communication circuit136 in order to conduct the far field communication sessions with theIMD 104. The far field communication circuit 136 includes transmittingand receiving capabilities and is coupled to an antenna for outputtingthe wireless far field signals 110 and for receiving the wireless farfield signals 112 sent by the IMD 104. These far field signals 110 maybe at a significantly higher frequency than the signals of the nearfield proximity communications 108.

FIG. 4 shows an example of components for embodiments of the IMD 104.The IMD 104 also includes a controller 140 that performs logicaloperations in order to establish the communication sessions with theexternal device 102. The controller 140 may also be of various formssuch as a general purpose programmable processor, an applicationspecific processor, hardwired digital logic, or combinations thereof.

The controller 140 communicates with other components of the IMD 104.For instance, the controller 140 may be linked to a therapy circuit 142that includes a pulse generator for producing electrical stimulationpulses. The therapy circuit 142 is in turn connected to a medical leadto deliver the electrical stimulation pulses to the target site withinthe body 118. The controller 140 may control the pulse generator of thetherapy circuit 142 in order to produce electrical stimulation signalsthat have characteristics specified by the therapy programming that hasbeen provided from the external device 102 to the IMD 104. For instance,the controller 140 may control which electrodes receive a stimulationsignal, the amplitude of the signal, the pulse width of the signal, andso forth.

The controller 140 also communicates with a proximity communicationcircuit 144 in order to receive the proximity communications 108including the unique key sent by the external device 102. The proximitycommunication circuit 144 may include a coil tuned to an appropriatefrequency for the near field, inductive coupling. The proximitycommunication circuit 144 may be unidirectional for receiving thesignals from the external device 102. Alternatively, the proximitycommunication circuit 144 may be bi-directional for embodiments wherethe external device 102 also has a bi-directional proximitycommunication circuit which allows the IMD 104 to send proximitycommunications back to the external device 102.

The controller 140 communicates with a far field communication circuit146 in order to conduct the far field communication sessions with theexternal device 102. The far field communication circuit 146 includestransmitting and receiving capabilities and is coupled to an antenna foroutputting the wireless far field signals 112 and receiving the wirelessfar field signals 110 from the external device 102.

FIG. 5 shows an example of logical operations that may be performed bythe external device 102 during the implantation procedure discussedabove in relation to FIGS. 1 and 2. Initially, the external device 102may open a communication session with an external stimulator (ENS) 120at an operation 152 to obtain patient and therapy information when theexternal stimulator 120 is present. The external device captures thedata from the external stimulator 120 at an operation 154. Additionallyor alternatively, the external device may obtain patient information andother information for setup of the IMD 104 by receiving information thatis manually entered by a clinician interacting with the user interfaceof the external device 102 at an operation 156.

While the IMD 104 is located outside of the sterile field, the externaldevice 102 establishes a far field communication session with the IMD104 at an operation 158. During the far field communication session, theexternal device 102 transfers setup data to the IMD 104 includingpatient information. The external device 102 then terminates the firstfar field communication session at operation 162.

The IMD 104 is then moved into the sterile field and is implanted intothe patient 118. The external device 102 then establishes a second farfield communication session with the IMD 104 at an operation 164. Theexternal device 102 programs the therapy settings into the IMD 104 andinstructs the IMD 104 to perform any integrity and performance testingat an operation 166. The IMD 104 then terminates the second far fieldsession at an operation 168 to conclude the implantation procedure.

The details of establishing the first and second far field communicationsessions as in operations 158 and 164 are shown in more detail in FIG.6. Initially, the external device 102 attempts to discover other devicesincluding the IMD 104 that are within far field communication range atan operation 170 by sending a discovery communication that provokes anydevice in range to respond. The external device 102 contemporaneouslyattempts to exchange a current, i.e. new, key with the IMD 104 via aproximity communication at an operation 172. This proximitycommunication may occur before, during, or immediately after thediscovery request.

Upon discovering the IMD 104, the external device 102 attempts to begina communication session by prompting the IMD 104 to respond where theexternal device 102 expects the response to use the current key that wastransferred at the operation 172. Where the operations of FIG. 6correspond to the operation 158 of FIG. 5 that pertains to the first farfield communication session, the current key is successfully transferredbecause the IMD 104 is present in close proximity to the wand or hoop106 outside of the sterile field. Therefore, the IMD 104 is able torespond with the appropriate key being used. In that case, the externaldevice 102 receives the response that is using the current key asexpected by the external device 102 such that the response is verifiedat a query operation 176. Operational flow then proceeds on to the nextoperation in FIG. 5.

The operations of FIG. 6 also correspond to the operation 164 of FIG. 5that pertains to the second far field communication session, where thecurrent key is not successfully transferred at operation 172. This isbecause the IMD 104 is no longer present in close proximity to the wandor hoop 106 outside of the sterile field but is instead within thesterile field. Therefore, the IMD 104 is not able to respond to theattempt by the external device 102 to begin the second communicationsession with the appropriate key. In that case, the external device 102does not receive a response that is using the current key as expected bythe external device 102 such that the response is not verified at thequery operation 176. Operational flow therefore proceeds on to anoperation 178.

At the operation 178, the external device 102 again attempts to begin acommunication session by prompting the IMD 104 to respond where theexternal device 102 now expects the response to use the last key thatwas used in the immediately prior communication session instead of thenew key. Where the operations of FIG. 6 correspond to the operation 164of FIG. 5 that pertains to the second far field communication session,the response by the IMD uses the last used key because the IMD 104 hasnot received any newer key and therefore continues to use the last usedkey when responding to the external device 102. In that case, theexternal device 102 receives the response that is using the last usedkey as expected by the external device 102 such that the response isverified at a query operation 180.

Upon the external device 102 verifying that the IMD 104 has respondedusing the last used key instead of the more current key, the externaldevice 102 outputs a warning about the communication session beingestablished with the last IMD at an operation 182. The external device102 may then prompt the clinician to confirm that the communicationsession with this last IMD should continue. This operation 182 ensuresthat the clinician does intend to have the communication session withthe last IMD rather than a different IMD. Where the clinician isattempting to program the IMD 104 shortly after the IMD 104 has beenimplanted such that the IMD 104 is indeed the last IMD that was in acommunication session with the external device 102, the clinician willselect to continue with the communication session since the last IMD isthe correct IMD for the second far field session. However, if theclinician sees this warning at operation 182 for an attempt at a firstcommunication session with a particular IMD, then the clinician willcancel the communication session because the last IMD is not the correctIMD for the session.

Returning to the query operation 180, if the attempt to start thecommunication session at operation 170 is not verified because noresponse is received that utilizes the last used key, then operationalflow proceeds to an operation 184. The external device 102 then providesa message to the clinician that suggests that the proximity wand or hoop106 be re-positioned because the IMD does not have either the currentkey or the last used key. The operations may then repeat. Where the IMD104 is still outside of the sterile field, which is likely to be thecase at this point since an IMD 104 inside the sterile field haspreviously received the key when outside the sterile field, theclinician adjusts that position of the wand or hoop 106 and the currentkey is then successfully transferred at the next instance of theoperation 172. The response is then able to be verified at operation176.

The operational flow of FIG. 5 is one particular example. In otherexamples, one or more communication sessions with the ENS may happen atany time where each communication session with the ENS is not treated asa communication session with an IMD such that the attempt to exchange anew key does not occur and the ability to attempt to use the last key isnot adversely affected. For instance, the far field session may firstoccur with the IMD 104, then the session with the ENS occurs, and then asubsequent far field session with the IMD 104 occurs and utilizes thesame key as the prior far field session with the IMD 104.

Furthermore, the process shown in FIGS. 5 and 6 is useful in othercontexts than during implantation. For instance, the same process may beuseful while the patient is in the recovery room and is positioned insuch a way that a near field communication cannot be performed.Therefore, the failed attempt to exchange a new key results in theexternal device 102 beginning a far field session using the last keythat was used with the IMD 104 during the implantation procedure.

As an additional safeguard, each key that is exchanged between theexternal device 102 and the IMD 104 may have a limited lifetime. The IMD104 may keep track of whether the key that the IMD 104 has received hasexpired. If the key has expired, then the IMD 104 does not use that keywhen responding to any attempt by the external device to start a farfield communication session. Therefore, any subsequent sessions withthat IMD requires a new key to be exchanged via a proximitycommunication.

While embodiments have been particularly shown and described, it will beunderstood by those skilled in the art that various other changes in theform and details may be made therein without departing from the spiritand scope of the invention.

What is claimed is:
 1. A method of communicating by an implantablemedical device, comprising: receiving a first key over a first proximitycommunication at the implantable medical device; receiving a first farfield communication at the implantable medical device in an attempt tobegin a first far field communication session with an external devicewhere the external device expects the implantable medical device to usethe first key when exchanging the communications during the first farfield communication session; sending from the implantable medical devicea response to the first far field communication where the response usesthe first key to begin conducting the first far field communicationsession; after sending the response, terminating the first far fieldcommunication session at the implantable medical device; afterterminating the first far field communications session, when a next farfield communication of a second far field communication session usingthe first key is received by the implantable medical device, thencommunicating by the implantable medical device during the second farfield communication using the first key; and after terminating the firstfar field communications session, when a second key over a secondproximity communication is received by the implantable medical device,then sending from the implantable medical device a response to the nextfar field communication of the second far field communication sessionusing the second key.
 2. The method of claim 1, wherein the implantablemedical device is configured to receive patient identificationinformation during the first far field communication session and receivetherapy programming information during the second far fieldcommunication session using the first key.
 3. The method of claim 1,wherein the implantable medical device discards the first key uponreceiving the second key.
 4. The method of claim 1, wherein the firstand second proximity communications are unidirectional.
 5. The method ofclaim 1, further comprising monitoring at the implantable medical devicean expiration period of the first key and discarding by the implantablemedical device the first key once the expiration period ends.
 6. Themethod of claim 1, further comprising terminating the second far fieldcommunications session.
 7. An implantable medical device, comprising: aproximity communication circuit that receives proximity communications;a far field communication circuit that sends and receives far fieldcommunications; and a controller configured to: receive a first key overa first proximity communication using the proximity communicationcircuit; receive a first far field communication using the far fieldcommunication circuit in an attempt to begin a first far fieldcommunication session with an external device where the external deviceexpects the implantable medical device to use the first key whenexchanging the communications during the first far field communicationsession; send a response to the first far field communication using thefar field communication circuit where the response uses the first key tobegin conducting the first far field communication session; aftersending the response, terminate the first far field communicationsession; after terminating the first far field communications session,when a next far field communication of a second far field communicationsession using the first key is received by using the far fieldcommunication circuit, then communicating by using the far fieldcommunication circuit during the second far field communication usingthe first key; and after terminating the first far field communicationssession, when a second key over a second proximity communication isreceived using the proximity circuit, then sending a response to thenext far field communication of the second far field communicationsession using the far field communication circuit and using the secondkey.
 8. The implantable medical device of claim 7, wherein thecontroller is further configured to receive patient identificationinformation during the first far field communication session and receivetherapy programming information during the second far fieldcommunication session using the first key.
 9. The implantable medicaldevice of claim 7, wherein the controller discards the first key uponreceiving the second key.
 10. The implantable medical device of claim 7,wherein the first and second proximity communications areunidirectional.
 11. The implantable medical device of claim 7, whereinthe controller is further configured to monitor an expiration period ofthe first key and discard the first key once the expiration period ends.12. A system, comprising: a first external device; and an implantablemedical device that comprises a proximity communication circuit thatreceives proximity communications, a far field communication circuitthat sends and receives far field communications, and a controllerconfigured to: receive a first key over a first proximity communicationfrom the first external device using the proximity communicationcircuit; receive a first far field communication from the first externaldevice using the far field communication circuit in an attempt to begina first far field communication session with the first external devicewhere the first external device expects the implantable medical deviceto use the first key when exchanging the communications during the firstfar field communication session; send a response to the first far fieldcommunication using the far field communication circuit to the firstexternal device where the response uses the first key to beginconducting the first far field communication session; after sending theresponse, terminate the first far field communication session; afterterminating the first far field communications session, when a next farfield communication of a second far field communication session usingthe first key is received from the first external device by using thefar field communication circuit, then communicating by using the farfield communication circuit during the second far field communicationusing the first key; and after terminating the first far fieldcommunications session, when a second key over a second proximitycommunication is received using the proximity circuit, then sending aresponse to the next far field communication of the second far fieldcommunication session using the far field communication circuit andusing the second key.
 13. The system of claim 12, further comprising asecond external device, and wherein the second proximity communicationcontaining the second key is sent by the second external device andwherein sending the response to the next far field communication of thesecond far field communication session using the far field communicationcircuit and using the second key is received by the second externaldevice.
 14. The system of claim 12, wherein the controller is furtherconfigured to receive patient identification information during thefirst far field communication session and receive therapy programminginformation during the second far field communication session using thefirst key.
 15. The system of claim 12, wherein the controller discardsthe first key upon receiving the second key.
 16. The system of claim 12,wherein the first and second proximity communications areunidirectional.
 17. The system of claim 12, wherein the controller isfurther configured to monitor an expiration period of the first key anddiscard the first key once the expiration period ends.